Redox Potentials and HPLC Behavior of Cobalt and Iron Complexes with Pyridylazo Compounds

The redox potentials of cobalt and iron complexes with ten pyridylazo compounds, E0ML2 (ML2+/0; M: CoIII/II, FeIII/II; L-: pyridylazo compounds), have been determined in order to explore the difference in their reversed-phase HPLC behavior. The redox potentials of Co complexes were in the range of -0.62 - 0.03 V, while those of Fe complexes were -0.06 - 0.59 V relative to 0.20 V for ferricinium/ferrocene. The redox potentials of both the Co and Fe complexes were linearly correlated to the basicities of the ligands. The correlation was quantitatively explained by a difference in dependence of the stabilities of MIII and MII complexes on the ligand basicities. The complex of [CoIIIL2]+ or [FeIIL2] with any compound injected in the reversed-phase HPLC system was detected without any change in the composition. When [CoIIL2] was injected, only those complexes having the highest potentials of E0CoL2 ≅ 0.0 V were detected as [CoIIL2], while other complexes having lower potentials gave a peak of [CoIIIL2]+. When [FeIIIL2]+ was injected, only complexes having the lowest potentials of E0FeL2 ≅ 0.0 V were detected as [FeIIIL2]+, while others having higher potentials gave a peak of [FeIIL2].